Directing the Morphology, Packing, and Properties of Chiral Metal–Organic Frameworks by Cation Exchange

Abstract We show that metal–organic frameworks, based on tetrahedral pyridyl ligands, can be used as a morphological and structural template to form a series of isostructural crystals having different metal ions and properties. An iterative crystal‐to‐crystal conversion has been demonstrated by consecutive cation exchanges. The primary manganese‐based crystals are characterized by an uncommon space group (P622). The packing includes chiral channels that can mediate the cation exchange, as indicated by energy‐dispersive X‐ray spectroscopy on microtome‐sectioned crystals. The observed cation exchange is in excellent agreement with the Irving–Williams series (MnZn) associated with the relative stability of the resulting coordination nodes. Furthermore, we demonstrate how the metal cation controls the optical and magnetic properties. The crystals maintain their morphology, allowing a quantitative comparison of their properties at both the ensemble and single‐crystal level.

OxfordSystems Cryosystem. Diffraction data for the Mn, Cu, Ni, and Co systems were collected with CuKα1 λ = 1.54184 Å measured on either a RigakuOD XtaLab pro X-ray diffractometer equipped with a Detris PilatusR 200K-A detector or on a Rigaku Synergy R with a HyPic-Arc 150°. The diffraction data for the Fe, Zn, and Co systems were collected with MoKα λ = 0.71073 measured on a Rigaku XtaLab Synergy diffractometer with a Pilatus 300K CdTe detector. The crystals were kept at 100K. Data were collected as ω scans of 0.5º frames with CrysAlisPro. The data were integrated and reduced using CrysAlisPro (Rigaku 2018). An absorption correction (either gaussian or multi-scan) was applied. The structures were solved by direct methods using SHELXT-2016/4 [S1] , as implemented in Olex2. [S2] The structures were fully refined with SHELXL-2016/4 [S1] All non-hydrogen atoms were refined with anisotropic displacement coefficients. Hydrogen atoms were placed in calculated positions and assigned isotropic displacement coefficients, and their coordinates were allowed to ride on the respective carbon atoms. The SQUEEZE protocol of Platon or the solvent masking routine of Olex2 was used. [S3] Alternative SCXRD refinement of the structures with Mn (no exchange) or Mn/M mixed occupancy metal sites (partial exchange) by allowing for the free refinement of metal composition occupancy (total occupancy was constrained to 1) was also examined. The resulting refinement R factors, goodness-of-fit, and the refinement stability of the structures were used to determine the final metal composition. For details, see Table S2 and the cif files v486 and v395, Table S3 for cif files v445 and v510, Table S4 for cif files v508 and v509, Table S5 for cif files v428-sq and v432, and Table S6 for cif files v430 and v433b.
Fluorescence spectroscopy. The crystals were drop-casted from methanol suspensions onto glass slides. The ligand (AdDB) was deposited by smearing. Next, the glass slides were placed vertically in the beam pathway. The sample was excited by a frequency tripled Nd:YAG Q-switched laser, pumping an optical parametric oscillator (Ekspla NT342/C/3/UVE), with a pulse duration of 5 nsec and a repetition rate of 10 Hz. The fluorescence spectra were collected at ~30º using a 20×0.4 NA objective, spectrally filtered using a color glass filter and a monochromator (Acton SpectraPro2150i) and were measured by a photomultiplier tube (Hamamatsu R10699). The photomultiplier transient output was measured by a 600 MHz digital oscilloscope (LeCroy Wavesurfer 62Xs). The laser beam pulse energy was measured by a pyroelectric sensor (PE9-C, Ophir Optronics). Lifetime measurements were measured as the above except that a 355 nm, <0.5 nsec Teem Photonics laser and a R5108 Hamamatsu PMT were used.
Measuring magnetic properties using a Superconducting Quantum Interference Device (SQUID). The measurements were carried out with a SQUID magnetometer MPMS3 (LOT-Quantum Design, Inc.) using the vibrating sample magnetometry (VSM) mode, applying a peak amplitude of 6 mm with a frequency of 13 Hz, and an average time of 5 s. The samples were mounted on a standard brass holder. Plots of the magnetic moments as a function of the applied magnetic field at constant temperatures of 5K and 300K are shown in Figure S9. The magnetic field was applied in intervals of T≤H≤+6T in both directions (H = magnetic field). Measurements of cT vs T, where c is the magnetic susceptibility of the samples, were performed in the temperature range of 2K-300K while applying a magnetic field of 0.5T (Mn-AdDB, Fe-AdDB, Co-AdDB, Ni-AdDB, Cu-AdDB), and 2.5 T (Zn-AdDB). The measurements were carried out using the field-cooled mode: briefly, the sample was cooled from room temperature in the same field that was used for measurements during the subsequent heating. The temperature dependencies of the magnetic susceptibility were normalized to moles using the chemical formula: C62H52MN4Cl2, were M = Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ or Zn 2+ . In the cT vs T plot, in order to separate the paramagnetic contribution from the molar magnetic susceptibility, we fitted the temperature dependencies by a Curie-Weiss (CW) equation in the form: c mol = C/(T-θ) + c0, where C = Curie constant, θ = Weiss parameter, c0 = contribution of diamagnetism from the orbital motion of electrons (temperature independent), and the samples impurities and holder. The dotted lines in Figure 7D, indicating the Curie constant values, were calculated: C = NAµ 2 B / 3kB g 2 S(S+1), where NA = Avagadro number, kB = Boltzmann constant, µB = Bohr magneton, S = spin quantum number of correspondent ion and g is the factor Lande taken as g = 2.
Cathodoluminescence Microscopy. Scanning electron microscopy (SEM), combined with cathodoluminescence (CL) spectra, was collected using a Gatan MonoCL4 Elite system equipped with a retractable diamond-turned mirror. The collected light was first imaged in panchromatic mode using a high-sensitivity PMT (photomultiplier tube) with a spectral range of l = 160-930 nm. The collected light was then directed to a monochromator and a charge-coupled device (CCD) for parallel spectroscopy. The spectral range was set to l = 300-800 nm with a band pass of 20 nm by choosing the 150 lines/mm grating centered on 550 nm and a 1 mm entrance slit. The CL system is installed on a Zeiss Gemini SEM 500, a high-resolution SEM equipped with a two-mode field emission gun. CL measurements were performed at 5kV with an aperture of 60 µm, and with high current in analytical gun mode. First, light and electron images were collected simultaneously. Then, CL spectra were collected on defined spots (pixel size 560 nm) that were marked on the pre-scanned SEM image. The acquisition time per spot was set to 20 sec for AdDB, Mn-AdDB, Fe-AdDB, Co-AdDB, Ni-AdDB, and Cu-AdDB and 10 sec for Zn-AdDB ( Figure S8). Spectra were collected on several crystals.
Solid-state UV spectra. UV absorbance spectra and the absolute PL quantum yield were collected by a HAMAMATUS Absolute PL Quantum Yield Spectrometer C11347 with a wavelength range of l = 370-850 nm. The instrument is equipped with a 150 W Xenon light source and an integrating sphere consisting of a 3.3 inch Spectralon. The samples were drop-casted from methanol solutions on a quartz petri dish and left to dry at room temperature. The samples were weighed before the measurements. The measurements were conducted between l = 370 to 800 nm with intervals of 10 nm. The quantum yields are reported in Table S7 for Zn-AdDB and AdDB.
Crystal-to-Crystal Conversion by Exchange of Mn 2+ by Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , and Zn 2+ . After the formation of Mn-AdDB, the mother liquid was removed with a syringe from the tube. Subsequently, a freshly prepared solution of FeCl2·4H2O (1.6 g, 8.2 mmol, and 400 mg/mL), CoCl2·6H2O (1.9 g, 8.2 mmol, and 485 mg/mL), NiCl2·6H2O (1.9 g, 8.2 mmol, and 484 mg/mL), CuCl2 (15 mg, 0.11 mmol, and 3.8 mg/mL), or ZnCl2 (15.6 mg, 1.15 mmol, and 3.9 mg/mL) in methanol (4.0 mL) was slowly added to the tube. The tube was sealed by Parafilm, tilted (70° from the base), and left at room temperature for two days. Subsequently, the solution was removed by a syringe. The colorless crystals left in a solution containing CuCl2 gradually became green. For the crystals immersed in solutions containing FeCl2·4H2O, CoCl2·6H2O, and NiCl2·6H2O, the intense color of the solutions prevented us from observing changes in the color of the crystals during the experiment. After isolation, the crystals appeared orange (FeCl2·4H2O), pink (CoCl2·6H2O), and light green (NiCl2·6H2O). The crystals remained colorless after the Mn 2+ to Zn 2+ exchange. Finally, the crystals were isolated, washed with methanol (4✕), and were stable at room temperature in methanol for six months.
Consecutive Crystal-to-Crystal Conversion by Metal Cation Exchange of Mn 2+ by Co 2+ , followed by the Exchange of Co 2+ by Cu 2+ . The mother liquid of Mn-AdDB was removed with a syringe from the glass tube. Subsequently, a freshly prepared solution of CoCl2·6H2O (1.9 g, 8.2 mmol, 485 mg/mL) in methanol (4.0 mL) was slowly added to the tube. The tube was sealed by Parafilm, tilted (70° from the base), and left at room temperature for 2 days. Subsequently, the solution was removed by a sy-ringe. Then, the crystals were washed four times using methanol. This step concludes the first metal exchange process; pink crystals of Co-AdDB were formed, as shown by SCXRD. The same sample of Co-AdDB was then immersed in a solution of CuCl2 (52.1 mg, 8.2 mmol, and 13.0 mg/mL) in methanol (4.0 mL). The tube was sealed by Parafilm, tilted (~70° from the base), and left at room temperature for 2 days. The sample underwent identical solvent removal and crystal washing processes as those previously described for the first metal exchanging process. This step concluded the second metal exchange process; green crystals of Cu-AdDB were formed, as shown by light microscopy and SCXRD.  Table S2. Single-crystal X-ray data and the structure refinement parameters for Mn-AdDB.  (2) Hexahedral (inner layer) and triangular channels handiness M P Hexahedral (outer layer) P M Table S3. Single-crystal X-ray data and structure refinement parameters after 2 days of exchange by CuCl2 at room temperature, Cu-AdDB.  (2) Hexahedral (inner layer) and triangular channels handiness M M Hexahedral (outer layer) P P Table S4. Single-crystal X-ray data and structure refinement parameters after 2 days of exchange by NiCl2 and CoCl2 at room temperature for Ni-AdDB and Co-AdDB.  (11) Hexahedral (inner layer) and triangular channels handiness P P Hexahedral (outer layer) M M Table S5. Single-crystal X-ray data and structure refinement parameters after 2 days of exchange by FeCl2 and ZnCl2 at room temperature for Fe-AdDB and Zn-AdDB.  (9) Hexahedral (inner layer) and triangular channels handiness M M Hexahedral (outer layer) P P Table S6. Single-crystal X-ray data and structure refinement parameters after 2 days of metal cation exchange from Mn-AdDB to Co-AdDB. Cu-AdDB was obtained after 2 days of metal cation exchange from Co-AdDB.    Figure S4. Experimental powder X-ray diffraction (PXRD) spectra of Mn-AdDB and the corresponding MOFs after metal cation exchange (black lines). The purple lines denote fits obtained using the single crystal X-ray data. Experimental intensity variations, due to the preferred orientation of the crystals on the surface, were considered in the fit by using spherical harmonic functions. [S4] The differences between the experimental intensity variations and their fits are denoted by brown lines. The goodness-of-fits are as follows: Mn-AdDB = 1.04; Fe-AdDB = 1.39; Co-AdDB = 1.82; Ni-AdDB = 1.42 and Cu-AdDB = 1.16.The values (a = b, c) are the estimated unit cell dimensions. Zn-AdDB was not sufficiently stable to measure the PXRD. Figure S9. Lifetime measurements using a l = 500 nm emission wavelength for AdDB (magenta), Mn-AdDB (yellow), and Zn-AdDB (green). The data were fitted by a mono exponential decay, deconvoluted from the system response function (RSF), shown by blue markers.